Prevent occasional deadlock (#1597)

Author: slozier

Src/IronPython/Runtime/PythonContext.cs

@@ -146,10 +146,13 @@ public sealed partial class PythonContext : LanguageContext {
         private int _tracebackListenersCount;
 
         internal FunctionCode.CodeList _allCodes;
-        internal readonly object _codeCleanupLock = new object(), _codeUpdateLock = new object();
+        internal readonly object _codeCleanupLock = new object();
+        internal readonly object _codeUpdateLock = new object();
         internal int _codeCount, _nextCodeCleanup = 200;
         private int _recursionLimit;
 
+        private readonly object _indexBindersLock = new object(); // https://github.com/IronLanguages/ironpython3/issues/1596
+
         internal readonly List<FunctionStack> _mainThreadFunctionStack;
         private CallSite<Func<CallSite, CodeContext, object, object>> _callSite0LightEh;
         private List<WeakReference> _weakExtensionMethodSets;
@@ -3579,7 +3582,7 @@ ref _binaryBinders[(int)operation],
                 Interlocked.CompareExchange(ref _getIndexBinders, new PythonGetIndexBinder[argCount + 1], null);
             }
 
-            lock (this) {
+            lock (_indexBindersLock) {
                 if (_getIndexBinders.Length <= argCount) {
                     Array.Resize(ref _getIndexBinders, argCount + 1);
                 }
@@ -3597,7 +3600,7 @@ ref _binaryBinders[(int)operation],
                 Interlocked.CompareExchange(ref _setIndexBinders, new PythonSetIndexBinder[argCount + 1], null);
             }
 
-            lock (this) {
+            lock (_indexBindersLock) {
                 if (_setIndexBinders.Length <= argCount) {
                     Array.Resize(ref _setIndexBinders, argCount + 1);
                 }
@@ -3615,7 +3618,7 @@ ref _binaryBinders[(int)operation],
                 Interlocked.CompareExchange(ref _deleteIndexBinders, new PythonDeleteIndexBinder[argCount + 1], null);
             }
 
-            lock (this) {
+            lock (_indexBindersLock) {
                 if (_deleteIndexBinders.Length <= argCount) {
                     Array.Resize(ref _deleteIndexBinders, argCount + 1);
                 }